1. Field of the Invention
The present invention relates to a photocatalyst with high photoactivity. More specifically, the present invention relates to photocatalyst particles and powder which are capable of exhibiting good photocatalytic function using a practical light source of extremely low intensity, such as a fluorescent lamp, and also relates to an organic polymer composition, a slurry, a coating agent and a film which displays both photocatalytic properties and hydrophilicity, incorporating such a photocatalyst, and articles using the same.
2. Description of Related Art
Conventionally, titanium oxide has been widely used as a typical, practical photocatalyst. Titanium oxide has the property of absorbing ultraviolet light at wavelengths below approximately 400 nm and producing an excited electron. When the generated electron and hole reach the particle surface, combinations with oxygen and water and the like generate a variety of different radicals. These radicals typically cause an oxidizing action, and oxidize and decompose substances adsorbed to the surface thereof. This is the basic principle of photocatalysis. The use of the optical functions of ultra fine particles of titanium oxide in antibacterial, deodorizing and stainproofing applications, and in environmental clean-up applications such as atmospheric purification and water quality purification are currently under investigation.
Examples of methods for maximizing catalytic function include the methods described below.
A method for,    (1) reducing the particle size (This method is extremely effective in suppressing the recombination of the generated electron and hole.);    (2) increasing the crystallinity (This method is effective in raising the speed with which the generated electron and hole diffuse towards the surface.);    (3) performing charge separation (This method involves charge separation of the generated electron and hole, to increase the yield of electrons and holes which reach the surface.); and    (4) adjusting of the band gap.
If the band gap is reduced (and the maximum wavelength of absorption is increased) by the addition of a minute quantity of an impurity, then the efficiency of light sources emitting little ultraviolet light, such as the sun and fluorescent lamps, can be improved.
Of these methods, in recent years, the investigation of so-called visible light responsive photocatalysts, aimed the method (4) above, has attracted considerable interest.
For example in Japanese Unexamined Patent Application, Laid-open No. Hei 9-262482, the maximum wavelength of light absorption for titanium dioxide was shifted to a longer wavelength by modification of an anatase titanium dioxide with high catalytic activity through ion injection of metal elements such as Cr (chromium) or V (vanadium), thereby producing a titanium dioxide capable of catalytic action under visible light irradiation. However, this type of ion injection of metal atoms requires a large apparatus and is expensive, meaning that the industrial practicality is limited.
In addition, Japanese Unexamined Patent Application, Laid-open No. 2001-72419 discloses a titanium oxide with an index X=B/A of no more than 0.97, wherein A represents the average of the half-width of the titanium peak at the first and second measurements among four measurements of the half-width of a titanium peak of titanium oxide with a bond energy within a range from 458 eV to 460 eV as measured by X-ray photoelectron spectroscopy, and B represents the average of the half-width of the titanium peak at the third and fourth measurements. However, not only is the activity of the powder unsatisfactory, but the powder is also colored, meaning that the potential applications of the powder are limited. In a practical sense, the powder also has other drawbacks, such as being unsuitable as a coating in which transparency is required.
Furthermore, many of the conventional visible light responsive photocatalysts require the use of a powerful light source such as a xenon lamp in order to ensure an adequate catalytic function, which, needless to say, reduces their practicality. A photocatalyst capable of exhibiting an adequate effect with a low cost light source, for example, a typical indoor light source such as a day white fluorescent lamp, would have considerable merit.
International Patent Application, No. WO94/11092 discloses a method of treating bacteria and malodorous substances by applying a photocatalytic thin film formed from a semiconductor such as titanium dioxide on the internal walls of a hospital ward or living spaces, although no mention is made of the method of producing activity within the titanium dioxide, nor of the photocatalytic activity of the particles. If normal titanium dioxide is used, then it is envisaged that the activity achieved using a light source with a very low proportion of ultraviolet light such as a fluorescent lamp would be even lower than that achievable using the visible light responsive photocatalyst described above.
Furthermore, a representative example of applications focusing on the photocatalytic function of fine particles of titanium oxide include methods of kneading fine particles of titanium oxide into a substrate such as an easy handling fiber or plastic molded product, or methods of applying fine particles of titanium oxide to a substrate such as cloth or paper. However, the powerful photocatalytic action of titanium oxide causes the decomposition not only of harmful organic materials and environmental pollutants, but also of the fiber, plastic or paper medium itself, meaning these types of medium are prone to deterioration and are unable to present a practical degree of durability. In addition, because of the ease of handling offered by fine particles of titanium oxide, paints comprising a mixture of fine particles of titanium oxide with a binder are being developed, but a durable and low cost binder capable of overcoming the above deleterious effects on the medium has yet to be found.
Japanese Unexamined Patent Application, Laid-open No. Hei 9-225319 and Japanese Unexamined Patent Application, Laid-open No. Hei 9-239277 disclose measures for suppressing or preventing the deterioration of a resin medium or a binder resulting from the powerful photocatalytic action of titanium oxide, and propose methods in which a photo-inactive compound, which comprises aluminum, silicon or zirconium, is supported on the surface of particles of titanium oxide in the form of islands with steric hindrance, thereby suppressing the photocatalytic action of the titanium oxide. However, although this method results in the supporting of islands of a photo-inactive compound on the photocatalyst, specific regions of the resin medium or binder are still exposed to the powerful photocatalytic action of the titanium oxide.
Japanese Unexamined Patent Application No. Hei 10-244166 (Laid-open No. Hei 11-335121) proposes a photocatalytic titanium oxide in which the surface of the titanium oxide has been coated with a film of porous calcium phosphate, although in this case, the coating of calcium phosphate causes a reduction in the photocatalytic performance of the catalyst.
International Patent Application, No. WO99/33566 discloses a fine particulate powder of titanium dioxide in which a porous calcium phosphate layer is formed on at least a portion of the surface of the titanium oxide particles, with an anionic surfactant provided at the interface therebetween.
Furthermore, Japanese Unexamined Patent Application, Laid-open No. 2002-1125 discloses a photocatalyst powder comprising fine particles of titanium dioxide containing an anionic active substance such as condensed phosphoric acid, wherein the interfacial potential of the fine particles in a water based environment of pH 5 is within a range from 0 to −100 mV.
In addition, in terms of slurries comprising titanium oxide with photocatalytic activity, Japanese Unexamined Patent Application, Laid-open No. Hei 11-335121 (Japanese Patent Application No. Hei 10-142008) discloses an anatase titanium oxide containing slurry produced by subjecting a titania sol solution, a titania gel or a titania sol/gel mixture to heat treatment and simultaneous pressure treatment inside a sealed vessel, and subsequently dispersing the product with ultrasonic waves or by mixing.
Furthermore, Japanese Unexamined Patent Application, Laid-open No. Hei 11-343426 discloses a photocatalyst coating with excellent dispersion stability, wherein the photocatalyst coating comprises titanium oxide with a Raman spectrum peak within a range from 146 to 150 cm−1 and in which the proportion of anatase titanium oxide is at least 95 mass %, and silica sol, in a solvent.
However, the isoelectric point of the titanium oxide is from 5 to 6, and at pH values close to neutral, namely from pH 5 to 9, the titanium oxide is prone to aggregation, and obtaining a stable, highly transparent dispersion (slurry, sol or the like) in a solvent is difficult. Accordingly, dispersions in the acidic region are typically used, although such dispersions have undesirable effects on living organisms and the environment, and exhibit a corrosive action on metals, which cannot be ignored, making the dispersions unsuitable for use on metal substrates. Consequently, a neutral, stable titanium oxide sol has been keenly sought.
Japanese Unexamined Patent Application, Laid-open No. Hei 11-278843 discloses a titanium oxide sol of pH 5 to 10, comprising 50 to 100 parts by weight of negatively charged titanium oxide colloidal particles component, 5 to 50 parts by weight of a chelating [complexing] agent, and 1 to 50 parts by weight of an alkaline component. Furthermore, Japanese Unexamined Patent Application, Laid-open No. 2000-290015 discloses a method of producing a neutral titania sol with transparency and dispersion stability in the neutral region and formed from deflocculated titanium oxide particles covered with a hydrated phosphate compound, by mixing a titania sol obtained by deflocculating hydrous titanium oxide with a water soluble titanium compound and a phosphate compound, and removing acid from the reaction liquid. In addition, Japanese Unexamined Patent Application, Laid-open No. Hei 7-89722 discloses a method in which a neutral titanium dioxide sol is stabilized with a hydroxycarboxylic acid or a derivative thereof, wherein prior to, during, or after the stabilization, the titanium dioxide sol is treated with a metal ion, an inorganic anion, a chelating [complexing] agent and/or an oxidizing agent.
As described above, a number of techniques have been disclosed, although the conventional technology to date has not been able to provide photocatalytic particles capable of exhibiting good photocatalytic function using a practical light source of extremely weak intensity such as a fluorescent lamp, while retaining good durability and dispersion stability in those cases when the photocatalyst is used with an organic material, nor an industrially useful method of producing a neutral, highly transparent slurry containing these types of particles.